The use of gas under pressure for driving free floating pistons to pump hydraulic fluid has long been known. Such pumps have been utilized to eliminate the necessity of employing motor driven pumps of the electrical type. The most pertinent prior art known to applicants is disclosed in U.S. Pat. No. 2,858,767, to N. V. Smith which employs a dual pump four-way control valve. Such apparatus is difficult to synchronize and to maintain operable under all load conditions. In addition thereto, since only two pistons are utilized the output fluid is pulsating.